Marijuana's effects on actual driving performance were assessed in a series of three
studies wherein dose-effect relationships were measured in actual driving situations that
progressively approached reality. The first was conducted on a highway closed to other
traffic. Subjects (24) were treated on separate occasions with THC 100, 200 and 300 g/kg,
and placebo. They performed a 22-km road tracking test beginning 30 and 90 minutes after
smoking. Their lateral position variability increased significantly after each THC dose
relative to placebo in a dose-dependent manner for two hours after smoking. The second
study was conducted on a highway in the presence of other traffic. Subjects (16) were
treated with the same THC doses as before. They performed a 64-km road tracking test
preceded and followed by 16-km car following tests. Results confirmed those of the
previous study. Car following performance was only slightly impaired. The third study was
conducted in high-density urban traffic. Separate groups of 16 subjects were treated with
100 g/kg THC and placebo; and, ethanol (mean BAC .034 g%) and placebo. Alcohol impaired
performance relative to placebo but subjects did not perceive it. THC did not impair
driving performance yet the subjects thought it had. These studies show that THC in single
inhaled doses up to 300 g/kg has significant, yet not dramatic, dose-related impairing
effects on driving performance.

INTRODUCTION

This article describes the results of a research program that was set up to determine
the dose-response relationship between marijuana and objectively and subjectively measured
aspects of real world driving; and to determine whether it is possible to correlate
driving performance impairment with plasma concentrations of the drug or a metabolite. The
program consisted of three driving studies in which a variety of driving tasks were
employed, including: maintenance of a constant speed and lateral position during
uninterrupted highway travel, following a leading car with varying speed on a highway, and
city driving. A laboratory study preceded the driving studies for identifying the highest
THC dose to be administered in the subsequent studies.

GENERAL PROCEDURES

Subjects in all studies were recreational users of marijuana or hashish, i.e., smoking
the drug more than once a month, but not daily. They were all healthy, between 21 and 40
years of age, had normal weight and binocular acuity, and were licensed to drive an
automobile. Furthermore, law enforcement authorities were contacted, with the volunteers'
consent, to verify that they had no previous arrests or convictions for drunken driving or
drug trafficking.

Each subject was required to submit a urine sample immediately upon arrival at the test
site. Samples were assayed qualitatively for the following common 'street drugs' (or
metabolites): cannabinoids, benzodiazepines, opiates, cocaine, amphetamines and
barbiturates. In addition a breath sample was analyzed for the presence of alcohol. Blood
samples were repeatedly taken after smoking by venepuncture. Quantitative analysis of THC
and THC-COOH in plasma was performed by gas chromatography/mass spectrometry (gc/ms) using
deuterated cannabinoids as internal standards.

Marijuana and placebo marijuana cigarettes were supplied by the U.S. National Institute
on Drug Abuse. The lowest and highest THC concentrations in the marijuana cigarettes used
in the studies were 1.75% and 3.57%, respectively. Subjects smoked the administered
cigarettes through a plastic holder in their customary fashion.

Subjects were accompanied during every driving test by a licensed driving instructor. A
redundant control system in the test vehicle was available for controlling the car, should
emergency situations arise.

In each study, subjects repeatedly performed certain simple laboratory tests (e.g.
critical instability tracking, hand and posture stability), estimated their levels of
intoxication and indicated their willingness to drive under several specified conditions
of urgency. In addition, heart rate and blood pressure were measured. Results of these
measurements are reported elsewhere (Robbe, 1994).

LABORATORY STUDY

Methods

Twenty-four subjects, equally comprised of men and women, participated in this study.
They were allowed to smoke part or all of the THC content in three cigarettes until
achieving the desired psychological effect. The only requirement was to smoke for a period
not exceeding 15 minutes. When subjects voluntarily stopped smoking, cigarettes were
carefully extinguished and retained for subsequent gravimetric estimation of the amount of
THC consumed.

Results

Six subjects consumed one cigarette, thirteen smoked two and four smoked three (data
from one male subject were excluded from the results because no drug was found in his
plasma after smoking). The average amount of THC consumed was 20.8 mg, after adjustment
for body weight, 308 g/kg. It should be noted that these amounts of THC represent both the
inhaled dose and the portion that was lost through pyrolysis and side- stream smoke during
the smoking process. There were no significant differences between males and females, nor
between frequent and infrequent users, with respect to the weight adjusted preferred dose.
It was decided that the maximum dose for subsequent driving studies would be 300 g/kg.

STUDY 1: DRIVING ON A RESTRICTED HIGHWAY

Methods

The first driving study was conducted on a highway closed to other traffic. The same
twelve men and twelve women who participated in the laboratory study served again as the
subjects. They were treated on separate occasions with marijuana cigarettes containing THC
doses of 0 (placebo), 100, 200, and 300 g/kg. Treatments were administered double- blind
and in a counterbalanced order. On each occasion, subjects performed a road-tracking test
beginning 40 minutes after initiation of smoking and repeated one hour later. The test
involved maintaining a constant speed at 90 km/h and a steady lateral position between the
delineated boundaries of the traffic lane. Subjects drove 22 km on a primary highway and
were accompanied by a licensed driving instructor. The primary dependent variable was the
standard deviation of lateral position (sdlp), which has been shown to be both highly
reliable and very sensitive to the influence of sedative medicinal drugs and alcohol.
Other dependent variables were mean speed, and standard deviations of speed and steering
wheel angle. Blood samples were taken 10 minutes before the driving tests (i.e. 30 and 90
minutes after initiation of smoking, respectively).

Results

All subjects were willing and able to finish the driving tests without great
difficulty. Data from one male subject were excluded from the results because no drug was
found in his plasma after smoking.

Figure 1 demonstrates that marijuana impairs driving performance as measured by an
increase in lateral position variability: all three THC doses significantly affected sdlp
relative to placebo (p STUDY 2: DRIVING ON A NORMAL HIGHWAY IN TRAFFIC

Methods

The second driving study was conducted on a highway in the presence of other traffic
and involved both a road-tracking and a car-following test. A new group of sixteen
subjects, equally comprised of men and women, participated in this study. A conservative
approach was chosen in designing the present study in order to satisfy the strictest
safety requirements. That is, the study was conducted according to an ascending dose
series design where both active drug and placebo conditions were administered, double-
blind, at each of three THC dose levels. THC doses were the same as those used in the
previous study, namely 100, 200, and 300 g/kg. Cigarettes appeared identical at each level
of treatment conditions. If any subject would have reacted in an unacceptable manner to a
lower dose, he/she would not have been permitted to receive a higher dose.

The subjects began the car-following test 45 minutes after smoking. The test was
performed on a 16 km segment of the highway and lasted about 15 minutes. After the
conclusion of this test, subjects performed a 64-km road-tracking test on the same highway
which lasted about 50 minutes. At the conclusion of this test, they participated again in
the car-following test. Blood samples were taken both before the first and after the last
driving test (i.e. 35 and 190 minutes after initiation of smoking, respectively).

The road-tracking test was the same as in the previous study except for its duration
and the presence of other traffic. The car-following test involved attempting to match
velocity with, and maintain a constant distance from a preceding vehicle as it executed a
series of deceleration/acceleration maneuvers. The preceding vehicle's speed would vary
between 80 and 100 km/h and the subject was instructed to maintain a 50 m distance however
the preceding vehicle's speed might vary. The duration of one deceleration and
acceleration maneuver was approximately 50 seconds and six to eight of these maneuvers
were executed during one test, depending upon traffic density. The subject's average
reaction time to the movements of the preceding vehicle, mean distance and coefficient of
variation of distance during maneuvers were taken as the dependent variables from this.

Results

All subjects were able to complete the series without suffering any untoward reaction
while driving. Data from one female subject were excluded from the results because no drug
was found in her plasma after smoking.

Road-tracking performance in the standard test was impaired in a dose- related manner
by THC and confirmed the results obtained in the previous closed highway study (Figure 2).
The 100 g/kg dose produced a slight elevation in mean sdlp, albeit not statistically
significant (p In the car-following test, subjects maintained a distance of 45-50 m while
driving in the successive placebo conditions. They lengthened mean distance by 8, 6 and 2
m in the corresponding THC conditions after 100, 200 and 300 g/kg, respectively. The
initially large drug-placebo difference and its subsequent decline is a surprising result.
Our explanation for this observation is that the subjects' caution was greatest the first
time they undertook the test under the influence of THC and progressively less thereafter.
The reaction time of the subjects to changes in the preceding vehicle's speed increased
following THC treatment, relative to placebo. The administered THC dose was inversely
related to the change in reaction time, as it was to distance. However, increased reaction
times were partly due to longer distance (i.e. the longer the distance to the preceding
vehicle, the more difficult it is to perceive changes in its speed). Statistical
adjustment for this confounding variable resulted in smaller and non- significant
increases in reaction time following marijuana treatment, the greatest impairment (0.32 s)
being observed in the first test following the lowest THC dose (Figure 3). Distance
variability followed a similar pattern as mean distance and reaction time; the greatest
impairment was found following the lowest dose. As in the previous study, plasma
concentrations of the drug were not related to driving impairment.

STUDY 3: DRIVING IN URBAN TRAFFIC

Methods

The program proceeded into the third driving study, which involved tests conducted in
high-density urban traffic. There were logical and safety reasons for restricting the THC
dose to 100 g/kg. It was given to a new group of 16 regular marijuana (or hashish) users,
along with a placebo. For comparative purposes, another group of 16 regular users of
alcohol, but not marijuana, were treated with a modest dose of their preferred
recreational drug, ethanol, and again placebo, before undertaking the same city driving
test. Both groups were equally comprised of men and women.

Marijuana was administered to deliver 100 g/kg THC. The driving test commenced 30
minutes after smoking. The alcohol dose was chosen to yield a bac approaching 0.05 g% when
the driving test commenced 45 minutes after onset of drinking. Active drug and placebo
conditions were administered double-blind and in a counterbalanced order in each group.
Blood samples were taken immediately prior to and following all placebo and drug driving
tests (i.e. 20 and 80 minutes after initiation of smoking, or 35 and 95 minutes after
initiation of drinking).

Driving tests were conducted in daylight over a constant 17.5 km route within the city
limits of Maastricht. Subjects drove their placebo and active- drug rides through heavy,
medium and low density traffic on the same day of the week, and at the same time of day.
Two scoring methods were employed in the present study. The first, a 'molecular' approach
adopted from Jones (1978), involved the employment of a specially trained observer who
applied simple and strict criteria for recording when the driver made or failed to make
each in a series of observable responses at predetermined points along a chosen route. The
second, a 'molar' approach, required the driving instructor acting as the safety
controller during the tests to retrospectively rate the driver's performance using a
shortened version of the Royal Dutch Tourist Association's Driving Proficiency Test. In
total, 108 items were dichotomously scored, as either pass or fail. Total test performance
was measured by the percentage items scored as 'pass'. Subscores were calculated for
vehicle checks, vehicle handling, traffic maneuvers, observation and understanding of
traffic, and turning'. This method has been applied previously to show the impairing
effects of alcohol and diazepam (De Gier, 1979; De Gier et al., 1981).

Results

Data from two male subjects in the marijuana group were excluded from the results
because neither THC nor THC-COOH was found in their plasma after smoking.

Neither alcohol nor marijuana significantly affected driving performance measures
obtained by the molecular approach, indicating that it may be relatively insensitive to
drug-induced changes. The molar approach was more sensitive. Table 1 shows that a modest
dose of alcohol (bac=0.034 g%) produced a significant impairment in city driving, relative
to placebo. More specifically, alcohol impaired both vehicle handling and traffic
maneuvers. Marijuana, administered in a dose of 100 g/kg THC, on the other hand, did not
significantly change mean driving performance as measured by this approach.

Subjects' ratings of driving quality and effort to accomplish the task were strikingly
different from the driving instructor's ratings. Both groups rated their driving
performance following placebo as somewhat better than 'normal'. Following the active drug,
ratings were significantly lower (35%, p Thus, there is evidence that subjects in the
marijuana group were not only aware of their intoxicated condition, but were also
attempting to compensate for it. These seem to be important findings. They support both
the common belief that drivers become overconfident after drinking alcohol and
investigators' suspicions that they become more cautious and self- critical after
consuming low doses of THC, as smoked marijuana.

Drug plasma concentrations were neither related to absolute driving performance scores
nor to the changes that occurred from placebo to drug conditions. With respect to THC,
these results confirm the findings in previous studies. They are somewhat surprising for
alcohol but may be due to the restricted range of ethanol concentrations in the plasma of
different subjects.

DISCUSSION

The results of the studies corroborate those of previous driving simulator and
closed-course tests by indicating that THC in inhaled doses up to 300 g/kg has
significant, yet not dramatic, dose-related impairing effects on driving performance (cf.
Smiley, 1986). Standard deviation of lateral position in the road-tracking test was the
most sensitive measure for revealing THC's adverse effects. This is because road-tracking
is primarily controlled by an automatic information processing system which operates
outside of conscious control. The process is relatively impervious to environmental
changes but highly vulnerable to internal factors that retard the flow of information
through the system. THC and many other drugs are among these factors. When they interfere
with the process that restricts road-tracking error, there is little the afflicted
individual can do by way of compensation to restore the situation. Car-following and, to a
greater extent, city driving performance depend more on controlled information processing
and are therefore more accessible for compensatory mechanisms that reduce the decrements
or abolish them entirely.

THC's effects on road-tracking after doses up to 300 g/kg never exceeded alcohol's at
bacs of 0.08 g%; and, were in no way unusual compared to many medicinal drugs' (Robbe,
1994; Robbe and O'Hanlon, 1995; O'Hanlon et al., 1995). Yet, THC's effects differ
qualitatively from many other drugs, especially alcohol. Evidence from the present and
previous studies strongly suggests that alcohol encourages risky driving whereas THC
encourages greater caution, at least in experiments. Another way THC seems to differ
qualitatively from many other drugs is that the former's users seem better able to
compensate for its adverse effects while driving under the influence.

Inter-subject correlations between plasma concentrations of the drug and driving
performance after every dose were essentially nil, partly due to the peculiar kinetics of
THC. It enters the brain relatively rapidly, although with a perceptible delay relative to
plasma concentrations. Once there, it remains even at a time when plasma concentrations
approach or reach zero. As a result, performance may still be impaired at the time that
plasma concentrations of the drug are near the detection limit. This is exactly what
happened in the first driving study. Therefore an important practical implications of the
study is that is not possible to conclude anything about a driver's impairment on the
basis of his/her plasma concentrations of THC and THC-COOH determined in a single sample.

Although THC's adverse effects on driving performance appeared relatively small in the
tests employed in this program, one can still easily imagine situations where the
influence of marijuana smoking might have a dangerous effect; i.e., emergency situations
which put high demands on the driver's information processing capacity, prolonged
monotonous driving, and after THC has been taken with other drugs, especially alcohol.
Because these possibilities are real, the results of the present studies should not be
considered as the final word. They should, however, serve as the point of departure for
subsequent studies that will ultimately complete the picture of THC's effects on driving
performance.

REFERENCES

De Gier JJ (1979) A subjective measurement of the influence of ethyl/alcohol in
moderate doses on real driving performances.Blutalkohol, 16, 363-370.